Controlled Synthesis And Electrochemical Properties Of Transition Metal Sulfide/Phosphide-based Electrode Materials

With the deterioration of eco-environment and the exhaustion of fossil fuels,the demand for sustainable energy is increasing and high-performing energy storage devices are becoming research hotspots.Common energy storage devices such as lithium-ion batteries and supercapacitors cannot meet the requirements of high energy density and high power density at the same time.Hybrid supercapacitor combines the advantages of battery and supercapacitor and has both high energy density and high power density,as well as excellent cycle life and high safety,which is a new type of energy storage device with great development potential.In recent years,as electrode materials for hybrid supercapacitors,transition metal sulfides/phosphides have been widely studied due to their high electrical conductivity and electrochemical activity.However,they still have some problems such as the sluggish reaction dynamics and insufficient reaction sites.Therefore,this work focuses on the control of material compositions,the design of morphologies and structures,increasing specific surface area,and optimizing the pore structure.And finally,the hybrid supercapacitors with high energy density and power density,excellent rate performance,and outstanding stability were fabricated.In this work,four transition metal sulfide/phosphide-based electrode materials were fabricated by simple synthetic methods.The relationships between their morphologies,phase structures and electrochemical properties were explored,also the energy storage mechanisms of the electrodes were illuminated.The main contents can be described as follows:(1)The self-supported FeCo2S4 nanosheets and petal-like Ni(OH)2 nanosheets were synthesized on the surface of nickel foam by hydrothermal methods.The fabricated FeCo2S4@Ni(OH)2 composite material has a three-dimensional(3D)layer-by-layer structure.The bimetallic sulfide FeCo2S4 nanosheets array has a great contribution to the high specific capacity.The introduction of Ni(OH)2 layer can protect the lower FeCo2S4 nanosheets from the erosion of the electrolyte,which could enhance the stability of the composite electrode material.And its 3D porous structure can effectively alleviate the volume change during the cycling tests.Therefore.the composite exhibits excellent electrochemical properties.The specific gravimetric capacity of FeCo2S4@Ni(OH)2 can up to 1342.8 C g-1 when the current density is 5 mA cm-2,and the capacity retention is 72%when the current density increases to 50 mA cm-2.The maximum energy density and power density of the assembled solid-state hybrid supercapacitor are 64 Wh kg-1 and 10.2 kW kg-1,respectively.After 10,000 cycles,the capacity retention rate is 92.9%and the coulombic efficiency is about 96.3%.(2)The phosphorus-doped Zn-Co sulfides which encapsulated in heteroatoms(N,S,and P atoms)doped carbon shell was fabricated by a simple room-temperature reaction and then two-step annealing method.Phosphorus-doped Zn-Co sulfides combine effectively with heteroatoms-doped carbon shells to produce a synergistic effect and then improve the electrochemical performance.Density functional theory simulations prove that phosphorus doping has a positive effect on improving the conductivity and reducing the adsorption energy of hydroxide ions,which is conducive to improving the electrochemical kinetics.The flexible electrode possesses excellent electrochemical performance.The maximum energy density and power density of the assembled flexible solid-state hybrid supercapacitor device are 62.9 Wh kg-1 and 16 kW kg-1,respectively.The capacity retention after 10,000 cycles is 92.0%at the bending and twisting state.(3)The nickel-iron phosphide electrode with porous nanosheets grown in-situ on nickel foam was synthesized by hydrothermal and annealing treatments.The effects of phosphating temperatures and doping amounts of Fe on the electrode morphologies,crystal phases,and electrochemical properties were investigated.Density functional theory calculation was also introduced to verify that the doping of Fe element and phosphorization could improve the conductivity and electrochemical kinetics.When the current density increases from 5 mA cm-2 to 50 mA cm-2,the specific gravimetric capacity of the phosphide electrode decreases from 1358 C g-1 to 799 C g-1.The assembled solid-state hybrid supercapacitor exhibits high energy density and high power density.The capacity retention rate is 91.5%and the coulombic efficiency is about 98.1%after 10,000 cycles.(4)3D self-supported nickel-cobalt phosphide electrode with controllable morphologies was prepared by two-step hydrothermal method and then annealing phosphating process.By using the precursor nanosheets as sacrificial templates to load a proper amount of Prussian blue analogue(PBA)on it.the agglomeration of PBA is prevented effectively.And the surface space of nickel foam is fully utilized,which could greatly increase the specific surface area.The energy storage mechanism of the nickel-cobalt phosphide electrode is mainly diffusion-controlled battery behavior,also accompanying by surface-controlled capacitive behavior.At the current density of 1 A g-1,the electrode delivers the specific gravimetric capacity as high as 1301 C g-1.The assembled solid-state hybrid supercapacitor device exhibits an energy density of 63.3 Wh kg-1 at the power density of 800 W kg-1 and can remain 96.4%of the initial capacity after 12000 cycles.This paper illustrates the feasibility of transition metal sulfide/phosphide-based materials as hybrid supercapacitor electrodes and provides a series of general synthetic methods for the preparation of high-performance electrode materials with excellent structure,which can be widely used in the field of future energy storage.